Numerous chromosomal gene shave been identified which are involved in retrovirus-induced disease and the transmission of retroviral genes in the inbred mouse. Genetic mapping of these genes to specific mouse chromosomes has been accomplished by the analysis of Chinese hamster x mouse somatic cell hybrids each of which contains different mouse chromosomes. In some cases, genes have also been mapped to subchromosomal regions or to specific map locations along these chromosomes by use of classical Mendelian crosses. These studies have led to chromosomal mapping of genes encoding the tumor rejection antigen gp96, the Fms and Met proto- oncogenes and a sequence which may represent the ecotropic MuLV cell surface receptor. The somatic cell hybrids have also been used to map other chromosomal gene for which cloned probes are available. Most recently, we have mapped genes for spermatid associated proteins, genes related to Drosophila heat shock proteins, P450 cytochromes, and several T lymphocyte antigens.